Dogberry, the magistrate of Messina, meets with Verges and the members of the night watch to appoint a new constable. He appoints a man who is literate. It is implied by his constant misuse of words that he himself is not. Dogberry begins to give the new constable and watchmen advice. In a comic and roundabout way, he suggests that they leave the criminals alone, reasoning that “they that touch pitch will be defiled.” (3.3.56-57) Verges remarks that Dogberry is a merciful magistrate, and Dogberry admits that he could not even bear to hang a dog. As he continues giving advice, Dogberry repeatedly misunderstands the meaning of words and the law. Finishing, he instructs the members of the watch to keep an eye on Leonato’s door—the big wedding is happening on the next day.
Dogberry’s malapropisms (misuses of words) and general lack of intelligence set up a major irony in the play: Dogberry, who has the information to stop (later, to uncover) Don John’s deception, is too incompetent and inarticulate to do so. This ties in with the play’s theme of language and the perception of reality. Dogberry’s inability to use language causes the other characters to misperceive reality. Dogberry’s comment about touching pitch is a good summary of how most characters in the play view shame: you can be tainted by even being close to someone who experiences it. Don Pedro’s fear of this is what lead him to agree to Don John’s scheme in the previous scene.
The members of the watch overhear Conrade and Borachio speaking as they shelter under a roof from the rain. Borachio brags that Don John has given him a thousand ducats for convincing Claudio and Don Pedro of Hero’s betrayal. For reasons which are unclear to Conrade, Borachio begins speaking about fashion, comparing it to a “deformed thief,” (3.3.131) which steals the true forms of young people, replacing them with fashionable dress, poses, and behavior. A night watchman misunderstands him, thinking he might be talking about a thief named “Deformed.” He and the other members of the night watch spring forth from the bushes and arrest the two conspirators for their “dangerous piece of lechery.” (3.3.167)
Because they are standing under a dripping roof, the members of the watch are literally “eaves-dropping.” The word originally meant to stand under the eaves extending outside a house, and listen to the conversation within. Borachio’s discussion of fashion relates to the theme of love and masquerade in the play—he is pointing out how important and deceptive appearances can be. The constable of the watch says “lechery,” but means to say “treachery.” Ironically, lechery (sexual lust) is what Borachio has caused Hero to be accused of.